Ukraine's President Volodymyr Zelenskiy said the government was considering several possible causes of a plane crash in Iran, which killed 176 citizens of seven countries.
The Ukraine International Airline jet crashed shortly after takeoff from Tehran on Wednesday. Among the victims were 82 Iranians, 63 Canadians, 11 Ukrainians, 10 Swedes, three Germans and three Britons.
In a television statement Zelenskiy asked people to refrain from manipulation, speculation, conspiracy theories and hasty evaluations regarding Iran plane crash. He also declared January 9 a day of national mourning.
He said that he would speak by telephone with the Iranian president to step up cooperation into the reason for the crash.
Four main theories, including missile attack
A top Ukrainian security official on Thursday set out what he said were the four main theories for why the Ukrainian airliner crashed, including a possible missile strike and terrorism.
Oleksiy Danylov, secretary of Ukraine's National Security and Defence Council, made his statement as an initial report by Iranian investigators said the plane, a Boeing 737-800, had been on fire immediately before it crashed.
Ukraine is looking at various possible causes of the crash, including a possible missile attack, a collision, an engine explosion or terrorism, he wrote in a Facebook post.
The crash happened hours after Iran launched missile attacks on US-led forces in Iraq, leading some to speculate that the plane may have been hit.
But an initial assessment by Western intelligence agencies was that the plane had suffered a technical malfunction and had not been brought down by a missile, five security sources - three Americans, one European and the Canadian - who asked not to be named, told Reuters.
Danylov said Ukrainian investigators in Iran wanted to search the crash site for possible debris of a Russian missile after seeing reports about its possible existence on the internet.
He referred to an unverified image being circulated on Iranian social media purportedly showing the debris of a Russian-made Tor-M1 surface-to-air missile of the kind used by the Iranian military.
"Our (investigative) commission is talking to the Iranian authorities about visiting the crash site and is determined to search for fragments of a Russian Tor air defence missile about which there was information on the internet," Danylov separately told Ukrainian news site Censor.net.
He said Ukraine would draw on expertise learnt from carrying out its own investigation into the 2014 shooting down of Malaysia Airlines flight MH-17 over eastern Ukraine, an incident that killed all 298 people on board.